Gas turbine engines generally have a plurality of axially aligned components including a fan, a compressor section, a combustor, and a turbine section. The fan, positioned at a forward end of the engine, rotates to draw in and accelerate ambient air. Some of the accelerated air flows downstream to the compressor section, as a core flow, where the air is compressed and then flows downstream to the combustor. At the combustor, the compressed air is mixed with fuel and combusted to form an exhaust. The exhaust expands from the combustor through the turbine section, causing turbines of the turbine section to rotate, and then flowing out of the engine at an aft end of the engine. The rotation of the turbines drives the rotation of the fan and compressors by way of a shaft, or a plurality of concentrically mounted shafts in the case of a multi-spool engine. It can therefore be seen that once this process has begun it is self-sustaining.
A reverse core gas turbine engine works differently in that air is pressurized with the fan at the forward end of the engine and transported to the aft end of the engine to the compressor section. From the compressor section, the air flows in a forward direction through the combustor and turbine section. From the turbine section, the air is redirected in an aftward direction to generate thrust. Typically, reverse core engines include three air streams, the core stream described above, a bypass stream that is utilized for only generating thrust, and a middle stream that mixes with the exhaust from the core stream before flowing out of the engine. If the fan pressure ratio between these streams is low, a variable fan intake nozzle may be necessary to control an operating line of the fan, especially between takeoff and cruise.
While effective, the equipment necessary for the variable fan intake nozzle introduces weight and complexity to the fan nozzle. Therefore, a new manner of controlling the fan operating line that does not introduce additional weight and complexity into the fan nozzle is necessary.